Field of the Invention
This invention relates to a catalyst carrier which is required to retain sufficient mechanical strengths at high temperatures, and to a process for producing such a carrier, and also to a method for the preparation of boehmite (Al.sub.2 O.sub.3.H.sub.2 O) from which said carrier is produced. More particularly, it relates to a catalyst carrier to be used in internal combustion engines for the purification of exhaust gases, and to a process for producing same.
Among conventional catalyst carriers for use in the purification of automobile exhaust gas, most of the granular types have been produced from .gamma.-alumina which is generally characterized by having a large specific surface, a small bulk density, and, nevertheless, high mechanical strengths. These conventional .gamma.-alumina catalyst carriers, however, transform into .alpha.-alumina on being exposed to high temperatures when in use and markedly lose in specific surface and in mechanical strengths, accompanied by increased abrasion and deterioration of the activity of catalyst. Moreover, in order to conform to the regulation for control of the NOx emission, recent tendency is to use in place of the former single bed oxidation catalyst a two-bed oxidation-reduction catalyst system which is placed nearer the engine in the exhaust pipe and, in some cases, even in the exhaust manifold, thus increasing the possibility of the above-noted troubles owing to exposure of the catalyst carrier to higher temperatures. Such troubles, originated from the changes in physical properties which take place along with the transformation of .gamma.-alumina to .alpha.-alumina, might be avoided, if a carrier having approximately the same properties as those of .gamma.-alumina could be produced from .alpha.-alumina having the most stable crystalline structure among the modifications of alumina. Known methods for producing an .alpha.-alumina carrier and the properties of the carrier obtained are as shown below.
(A) Aluminum hydroxide (Al.sub.2 O.sub.3.3H.sub.2 O), used as starting material, is calcined at 500.degree. to 700.degree. C. to form an activated alumina (.gamma.-alumina, etc.), and the powdered activated alumina is granulated after addition of a binder or the like, and then sintered by heating at a higher temperature of 1,400.degree. to 1,600.degree. C. to obtain an .alpha.-alumina carrier having high mechanical strengths.
(B) The same starting material as in (A) is calcined at a high temperarture of 1,200.degree. to 1,300.degree. C. to form an .alpha.-alumina powder, the resulting powder is granulated after addition of a binder or the like, and the granules are sintered by heating at a higher temperature of 1,400.degree. to 1,600.degree. C. to obtain an .alpha.-alumina carrier of high mechanical strengths.
Upon examination under an electron microscope, the present inventors found that both of the catalyst carriers prepared by the above methods (A) and (B) are composed of .alpha.-alumina particles in regular form of hexagon or sphere of the even size and, moreover, the particles are arranged closely packed together. This particle form seems to be the main cause of the aforesaid troubles. The reason may be explained in the following manner by taking the case (B) as example. Under an electron microscope, the particles of aluminum hydroxide used as starting material showed a regular hexagonal form of the even size, and the particles after having been calcined at 1,300.degree. C. were also of regular forms of a shrinked hexagon and an approximate sphere, each of the even size. After granulation and subsequent sintering, the particles may be bonded together through plane contact or spheric contact because of the above-noted shape of the particles. Consequently, in order to develop sufficient strength, the particles must be firmly bonded together by sintering at quite a high temperature or by use of a powerful binder, resulting in decreased specific surface, increased bulk density, and decreased pore volume. The decreased specific surface of the carrier causes difficulty in supporting the catalyst (for example, platinum or palladium), while the increased bulk density or decreased pore volume results in increased heat capacity and makes it difficult to obtain a satisfactory initial activity of the catalyst for exhaust gas purification. Therefore, the properties of .alpha.-alumina catalyst carriers prepared by the known methods are quite different from those of .gamma.-alumina required for a catalyst carrier to be used in automobile exhaust gas purification. Actually, no .alpha.-alumina catalyst is used at present in automobile exhaust gas purification.
On the other hand, boehmite (Al.sub.2 O.sub.3.H.sub.2 O), which constitutes the base material for the catalyst carrier, has heretofore been prepared by hydrothermal treatment (a treatment by contacting with water vapor at high temperatures and high pressures) of powdered aluminum hydroxide having the chemical formula Al.sub.2 O.sub.3.3H.sub.2 O which is used as raw material without any pretreatment.
However, in carrying out the conventional method for preparing boehmite by hydrothermal treatment of powdered aluminum hydroxide used as raw material without any pretreatment, difficulty is encountered in penetration of the water vapor into pores between particles owing to inevitable compact loading of aluminum hydroxide powder in the autoclave. Consequently, when a large quantity of aluminum hydroxide powder is to be converted into boehmite, with the depth of the charged layer of aluminum hydroxide powder, penetration of water vapor becomes more difficult and, hence, the conversion to boehmite is more retarded owing to insufficinet contact with the water vapor. For the above reason, the conventional method for preparing boehmite from aluminum hydroxide powder without any pretreatment has disadvantages with respect to quantity, time, and efficiency.